The present invention relates to an improved method for operating an internal combustion engine, in particular to an improved method for starting an internal combustion engine, and to an internal combustion engine adapted to operate under such a method.
Internal combustion engines find use in a wide range of applications. One particular application is the stationary internal combustion engine, used as a source of power at sites, such as oil well heads and drilling operations. It is in this application that internal combustion engines powered by gaseous hydrocarbons, in particular natural gas, have become prominent.
Typically, such engines operate using fuels, such as diesel, which are ignited under compressive conditions generated within the cylinders of the engine, or using fuels, such as gasoline or natural gas, which require the provision of an ignition source. Commonly, the source for the ignition in the latter type of engine is a spark generated in the combustion of the engine, most commonly by means of a spark plug. In order to improve the durability and rating of the spark-ignited engines, the so-called xe2x80x9cpilot ignitedxe2x80x9d engines have been developed. In the pilot ignited engines, a small volume of a liquid fuel having a suitably high cetane number is injected directly into the combustion chamber. The volume of fuel so injected is typically about 5 percent of the total fuel volume. The pilot ignited engines have found acceptance in the industry as a result of the aforementioned improvements. In addition, these engines offer the ability to change the fuel composition during the running of the engine and to switch, for example, from natural gas to diesel operation, without modification or shutting down of the engine. These engines are referred to as xe2x80x9cdual fuelxe2x80x9d or xe2x80x9cgas dieselxe2x80x9d engines.
Examples of dual fuel engines are disclosed in U.S. Pat. Nos. 4,603,674, 4,463,734, and 4,527,516. In each of these documents, an internal combustion engine is disclosed, which can operate on a mixture of fuels introduced directly into the combustion chamber of the engines.
As a result of concerns regarding emissions from internal combustion engines, a system was developed, in which the engine is provided with a so-called xe2x80x9cauto-ignition chamberxe2x80x9d or xe2x80x9ctorch cellxe2x80x9d. In such an arrangement, the engine is provided with a pre-combustion chamber in communication with the combustion chamber of the engine. A combustible fuel mixture is introduced into the pre-combustion chamber and ignited therein. The thus ignited fuel mixture is then allowed to enter the combustion chamber, where it ignites the main fuel/air mixture. An engine of this configuration is able to operate using a lean fuel mixture, which in turn results in a reduction in the emissions from the engine. Examples of such engines are as follows.
U.S. Pat. No. 4,306,526 discloses an internal combustion engine and a method for its operation. The engine is a turbocharged, two cycle engine and is operated on a lean fuel/air mixture. A jet cell ignition device is provided to ensure uniform and complete combustion of the fuel mixture. U.S. Pat. No. 4,966,103 discloses a combustion system for an internal combustion engine. The engine is a dual fuel engine operating on a mixed gas-liquid fuel and comprises a torch cell comprising an auto-ignition chamber in which a fuel having a sufficiently high cetane rating is caused to ignite. This in turn ignites the charge of fuel and air in the main combustion chamber of the engine.
A number of other torch-ignition systems are also known. U.S. Pat. No. 4,127,095 discloses a torch-ignition internal combustion engine provided with a main and pre-combustion chamber, in communication with each other through a torch aperture. The electrodes of a spark plug are disposed within the pre-combustion chamber. During operation, a lean air-fuel mixture is introduced into the main combustion chamber, from where a portion enters the pre-combustion chamber under the action of the compression stroke of the engine. The combustible mixture in the pre-combustion chamber is ignited by the spark plug, the flames from which propagate through the torch aperture into the main combustion chamber and thereby act as an ignition source for the main portion of the fuel mixture.
A modified version of this torch-ignition system is disclosed in U.S. Pat. No. 5,947,076. This patent describes and internal combustion engine having a main combustion chamber and a pre-combustion chamber, as in U.S. Pat. No. 4,127,095. The pre-combustion chamber is again provided with a spark plug. However, in the embodiment of U.S. Pat. No. 5,947,076 the electrode of the spark plug is located within a further chamber, a so-called xe2x80x9cplug combustion chamberxe2x80x9d, intended to protect the electrode of the spark plug from turbulence within the main and pre-combustion chambers.
U.S. Pat. No. 5,293,851 addresses the issue of providing an improved internal combustion engine, in particular engines operating with gas as a fuel. One issue that can arise with such engines is problems with starting the engine, in particular from cold. Typically, in order to improve the starting of the engine, it was known to add a certain amount of diesel fuel to the gas fuel during the start-up phase, until the engine could be run on gas alone. In order to reduce this problem, U.S. Pat. No. 5,293,851 provided the internal combustion engine with a torch cell having a pre-combustion chamber provided with a glow plug. In operation, a combustible fuel mixture, typically and air-gas mixture, is introduced into the main combustion chamber of the engine. During the compression stroke of the engine, a portion of the compressed mixture is forced into the pre-combustion chamber of the torch cell. At or about the same time, a liquid fuel, typically diesel, is injected directly into the pre-combustion chamber so as to contact the glow plug. The action of the glow plug is to ignite the liquid fuel, which in turn ignites the gaseous fuel within the pre-combustion chamber. The flame front leaves the pre-combustion chamber and enters the main combustion chamber and ignites the main fuel mixture. The engine of U.S. Pat. No. 5,293,851 is capable of operating under reduced compression ratios, in turn allowing it to accommodate fuels having a lower octane rating than can be accommodated by the higher compression ratios of the more conventional torch-ignited engines.
The engine of U.S. Pat. No. 5,293,851 has been found to work very well and provide a range of advantages. However, some difficulty has been encountered with the operation of the engine under certain conditions. In particular, it has been found that starting the engine can be troublesome, especially when the engine is cold. It has been found that the walls of the pre-combustion chamber, when cold, act as a sizeable heat sink and impair the combustion characteristics of the engine, leading to poor starting performance. In addition, it has been found that, under certain conditions, the glow plug can provide a source for pre-ignition of the air-fuel mixture. This in turn leads to poor running characteristics of the engine and, if allowed to persist, can lead to extensive damage to the engine components.
Accordingly, there is a need for an internal combustion engine arrangement which provides all the advantages of the engine of U.S. Pat. No. 5,293,851, but which does not suffer from the aforementioned drawbacks.
Surprisingly, it has been found that the running problems of the engine of U.S. Pat. No. 5,293,851 are significantly improved if a spark ignition system is provided for the combustible fuel mixture in the main combustion chamber, in addition to the torch-cell ignition system. Under difficult running conditions, such as cold starting, the spark ignition is used to supplement the torch-cell ignition system and achieve efficient operation of the engine.
Accordingly, the present invention provides a method for operating an internal combustion engine, which engine comprises a combustion chamber defined by the inner wall of a cylinder, a cylinder head and the upper surface of a piston moveable longitudinally within the cylinder, the engine further comprising a pre-combustion chamber in communication with the combustion chamber, the method comprising:
(a) supplying to the combustion chamber a first hydrocarbon fuel and oxygen to form a first combustible mixture;
(b) supplying to the pre-combustion chamber a second hydrocarbon fuel and oxygen to form a second combustible mixture, the second combustible mixture being auto-ignitable; and
(c) compressing the first and second combustible mixtures by moving the piston within the cylinder;
the method further comprising the step of:
(d) generating a spark to cause ignition of the compressed first combustible mixture in the combustion chamber.
As noted, the method of this invention is particularly well suited to the cold starting of the internal combustion engine. Accordingly, in a further aspect, the present invention provides a method for starting an internal combustion engine, which engine comprises a combustion chamber defined by the inner wall of a cylinder, a cylinder head and the upper surface of a piston moveable longitudinally within the cylinder, the engine further comprising a pre-combustion chamber in communication with the combustion chamber, the method comprising:
(a) supplying to the combustion chamber a first hydrocarbon fuel and oxygen to form a first combustible mixture;
(b) supplying to the pre-combustion chamber a second hydrocarbon fuel and oxygen to form a second combustible mixture, the second combustible mixture being auto-ignitable;
(c) compressing the first and second combustible mixtures by moving the piston within the cylinder; and
(d) generating a spark to cause ignition of the compressed first combustible mixture therein;
the step of generating a spark in the combustion chamber being carried out until the engine will run using auto-ignition of the second combustible mixture to ignite the first combustible mixture alone, thereafter ceasing the step of generating a spark.
The spark ignition system is only employed during those periods, such as start-up, when the operating conditions of the engine are such that operation with the torch-ignition system alone relying upon auto-ignition of the fuel mixture in the pre-combustion chamber would be inadequate for proper running of the engine. Constant operation of the engine using the spark ignition system would result in a high rate of wear and erosion of the electrodes of the spark plug. This would eventually result in poor engine performance and the need for frequent maintenance. Accordingly, it is most desirable that the use of the spark ignition system is kept to the minimum level necessary to achieve optimum engine performance.
The spark may be generated in the combustion chamber, causing direct ignition of the first combustible mixture. Alternatively, the spark may be generated in a spark-combustion chamber in communication with the combustion chamber. The spark-combustion chamber may be charged with a hydrocarbon fuel and oxygen to form a third combustible mixture of a hydrocarbon fuel, which is ignited at the appropriate time in the cycle of the engine. Alternatively, the spark-combustion chamber is filled with a portion of the first combustible mixture under the action of the compression stroke of the engine. Generation of a spark in the spark-combustion chamber ignites the combustible mixture in the spark-combustion chamber, which in turn causes the first combustible mixture in the combustion chamber to ignite. The use of a separate spark-combustion chamber in this manner can serve to protect the electrodes of the spark plug from high rates of wear.
In still a further aspect, the present invention provides an internal combustion engine comprising:
a combustion chamber defined by the inner wall of a cylinder, a cylinder head, and the upper surface of a piston moveable longitudinally within the cylinder, the combustion chamber having at least one inlet for the introduction into the combustion chamber of a first hydrocarbon fuel and oxygen and an outlet for exhaust gases;
a pre-combustion chamber having an inlet for the introduction into the pre-combustion chamber of a second hydrocarbon and a passageway in communication with the combustion chamber; and
a spark generator operable, so as to generate a spark capable of causing ignition of the first combustible mixture within the combustion chamber.
The spark generator may be located so as to generate a spark directly in the combustion chamber, thereby directly igniting the first combustible mixture in the combustion chamber. Alternatively, the spark generator comprises an electrode, the electrode being located within a spark-combustion chamber, the spark-combustion chamber being in communication with the combustion chamber, whereby a spark-ignited combustible mixture in the spark-combustion chamber may enter the combustion chamber. The spark-combustion chamber may be provided with one or more inlets for a hydrocarbon fuel and oxygen to form a third combustible fuel mixture. Alternatively, the spark-combustion chamber may be arranged to as to be charged with a portion of the first combustible mixture from the combustion chamber, during the compression stroke of the engine.
In still a further aspect, the present invention includes the provision of a cylinder head, which may be used to modify existing torch-ignition engines by replacing the existing cylinder head, thereby allowing the engine to be operated, in particular started, using the methods of this invention. Accordingly, the present invention provides a cylinder head for an internal combustion engine, the cylinder head comprising:
a combustion chamber face, which face, when the cylinder head is mounted on an engine block of an engine, together with the inner walls of a cylinder and the upper surface of a piston moveable longitudinally within the cylinder, defines a combustion chamber;
a pre-combustion chamber, having an inlet for a hydrocarbon fuel;
a passageway having an opening in the pre-combustion chamber and extending from the pre-combustion chamber to an opening in the combustion face; and
a receptor for a spark plug, the receptor being positioned so that the electrode of a spark plug located in the receptor is able to cause ignition of a combustible mixture present in the combustion chamber, when the cylinder head is mounted on an engine.
The cylinder head may have the receptor positioned so that the electrode of a spark plug located in the receptor is in direct contact with gases present in the combustion chamber, when the cylinder head is mounted on an engine. Alternatively, the cylinder head may comprise a spark-combustion chamber, the receptor being positioned so that the electrode of a spark plug located in the receptor extends into the spark-combustion chamber, the spark-combustion chamber being in communication with the combustion chamber when the cylinder head is mounted on an engine. The spark-combustion chamber may be provided with one or more inlets for a hydrocarbon fuel and oxygen. Alternatively, the spark-combustion chamber may be arranged so as to be charged with a portion of the first combustible mixture from the combustion chamber, during the compression stroke of the engine.
The first hydrocarbon fuel is preferably a gaseous hydrocarbon, that is a hydrocarbon that is normally gaseous under normal temperature and pressure. Natural gas is a most convenient fuel for use as the first hydrocarbon fuel.
The second hydrocarbon fuel may be any suitable fuel that is compressively ignitable, that is will ignite under compression during the compression stroke of the engine. This is in contrast to the first hydrocarbon fuel, which is one that, generally, will not ignite under compression alone and requires an ignition source, such as a spark or a torch-cell. The second hydrocarbon fuel is preferably a normally liquid hydrocarbon. A most suitable fuel for use as the second hydrocarbon fuel is diesel.
The method of the present invention may be applied to two cycle, as well as four cycle engines, with a four cycle engine being preferred.